The transmission of serial data at a determined data transmission rate is quite common in a multitude of technical areas. For example, a data source such as a microcontroller, may transmit such serial data at a determined first clock signal or frequency through a unidirectional data connection. Such a data connection may, for example, be a single wire bus without any response capability. The transmitted data are received by a receiver such as an integrated circuit (IC) and are to be evaluated within the receiver. Customarily a transmitter as the source of the data signals and the receiver do not have a common synchronizing clock. Therefore, the receiver must itself synchronize to the transmitter for recovering the first clock signal that is contained within the serial data. Such clock signal retrieval is necessary in order to correctly interrogate the data in a bit-by-bit fashion.
Normally the signal rate of the first clock signal is represented by a fraction of a local oscillator frequency or local oscillator clock rate of the receiver, whereby the local oscillator clock rate is provided as a second clock rate or frequency. Ideally, the first and second clock frequencies should be the same, whereby synchronization is automatically assured. In practice the local oscillator clock rate is determined, as a rule, internally of the integrated circuit by a local (RC)-oscillator having a clock frequency that is known only within certain limits. With the beginning of a starting bit of a data signal a counter is started to count. The respective count is incremented with each clock period or cycle, that is, with each internal or local clock pulse. The counter is reset to zero and then begins to count again when a count is reached that corresponds to a division ratio between the first data clock signal of the received signal and the second local oscillator clock signal or first and second clock signal frequencies. This resetting corresponds, when a correct synchronization is achieved, exactly to the beginning of the next data bit. Exactly at the center of the division, that is in the center of the data bit, the bus level is interrogated and thus the respective bit read-out. A second counter terminates this operation when a certain number of bits has been read.
Conventionally, there is the basic problem that the reading operation fails due to an erroneous synchronization between transmitter and receiver. A failed read-out can have grave adverse effects. Particularly in connection with uni-directional transmission media, there is no response possibility. Therefore, the receiver cannot request a known synchronization signal for a renewed follow-up synchronization during the transmission. Furthermore, it is not possible to make a renewed request for a data information that has been erroneously recognized. An erroneous synchronization means in this context that the divider or rather the division ratio deviates from a correct value either upwardly or downwardly. As a result, the counter is not reset as required, namely at the end of a data bit. Rather, the counter is reset either somewhat too early or somewhat too late. Correspondingly, the interrogation of the bus level does not take place at the center of a bit. To make matters worse, the up or down shifts accumulate as the number of bits to be interrogated increases. When the last bit of a sequence of bits is still correctly interrogated, the system is tolerant relative to an erroneous synchronization. Otherwise, double interrogations may be necessary and/or individual bits may be skipped. Further, the second clock frequency of the receiver is known only within certain limits such as ±10% of a rated clock frequency. Such tolerances are necessary due to manufacturing conditions. As a result, it is not possible to set a fixed divider or division ratio to achieve a constant, correct synchronization of the transmitter and the receiver forming together a system.
In order to achieve a certain synchronization between a transmitter and a receiver it has been suggested in conventional data transmission systems such as the LIN-bus (Local Interconnect Network) to transmit a so-called synchronization field. This synchronization field is transmitted at uniform time spacings prior to each data transmission and subsequent to a synchronization pause having a predetermined minimal duration. Reference is made in this connection to “LIN Protocol Specification”, Revision 1.1 2000. The suggested system has the particular disadvantage that only a limited data transmission rate is available for useful data due to the necessary transmission interruptions. Furthermore, a single transmission error during the synchronization field, leads to a faulty synchronization during the following data transmission.
Another disadvantage of the above system is seen in that the clock frequency of the receiver can change during the operation, for example due to temperature influences. Thus, there is the possibility that, even though a single synchronization was correct, a following evaluation may be erroneous, nevertheless.